This application claims the benefit of priority under 35 U.S.C. xc2xa7119 of European Patent Application Serial No. EP01114842.6 filed on Jun. 28, 2001.
1. Field of the Invention
The invention relates to an opto-electronic device module for use in optical telecommunications.
2. Technical Background
In high-power opto-electronic devices, such as laser diodes, the device is generally fixed on a submount made of a material with high thermal conductivity, such as diamond, SiC or AlN. The submount is bonded onto a circuit board, which can accommodate all or part of the complementary internal devices of the laser package, such as a thermistor, a back-field photodetector and circuit patterns.
Optical components for coupling light signals to or from the device, in particular optical fibres and lenses, are fixed in optically aligned position in front of the laser with the help of holding and aligning members (e.g., ferrule, support tube, saddle or clip, or welding blocks). Holding and aligning members and optical components are generally mounted on a base plate, which is also referred to as optical bench. An accurate optical alignment of the optical fibre to the opto-electronic device, e.g., laser, is required in order to maximise the optical coupling efficiency, or, in other words, to maximise the optical output power. For coupling of monomode fibres, submicron fibre alignment tolerances, typically less than about 0.1-0.2 xcexcm, are necessary. In addition, the alignment ought to be maintained within minimal variations over the course of time and against temperature deviations. Laser welding is the common process of choice for fixing the holding and aligning members. Laser welding occurs by local heating of the joint parts by means of a focussed laser beam. When highly accurate alignment is required, care should be taken in order to minimise post weld shift (PWS) which results from contraction of a cooling laser weld spot. To minimise displacement shifts, optical members and welding platform are generally made of a material with low thermal expansion coefficient. To this purpose, Kovar, stainless steel or Invar are often preferred.
U.S. Pat. No. 6,184,987 describes a process for laser welding a ferrule of a fiber optical cable to a clip of a fiber module. The disclosed process detects and corrects a movement of the fibre optic cable so that alignment is achieved. The process comprises moving a ferrule with an automated device after a weld is completed and then detecting a change in optical power of the light beam that is transmitted through the optical fibre. The detection can determine a direction along which the ferrule shifted during the weld process and subsequent laser welds of the ferrule can correct the shift.
Article xe2x80x98Laser Diode Packaging Technology: 980 nm EDFA Pump Lasers for Telecommunication Applicationsxe2x80x99 by Mobarhan K. S. and Heyler R., that was available on Oct. 24, 2000, in the Internet at the URL address http://www.newport.com/Support/Application_Notes/APPNOTES3.pdf, describes an optical subassembly that is a miniature aluminium nitride based optical bench containing the laser chip, laser diode submount, photodiode, and various other components. The optical subassembly also includes a relatively large metal platform onto which all the fiber pigtailing components are welded.
In PCT patent application WO 97/05513 a laser diode is placed on a substrate of alumina, which carries also different connections and other electronic components. The circuit board is mounted onto a metallic baseplate, preferably made of an alloy of chromium-nickel, such as Kovar. The aligning and fixing members comprise either an inverted U-shaped member placed on a cylindrical ferrule or a ferrule having two lateral vertical faces. The aligning and fixing members are made of the same material as the baseplate to allow easier laser welding and to minimise the differential expansion between the parts.
U.S. Pat. No. 5,570,444 describes a method of optically coupling optical fibres to injection lasers. A heat-sink carrying a laser diode is mounted on a Kovar substrate. An optical fibre with a lensed end is secured by means of solder to an elongated support member, which has the form of a slotted cylindrical rod. The end of the slotted rod close to the lensed end of the fibre is secured by laser welding to two slide members. The end of the rod remote from the fibre lensed end is secured, always by laser welding, to a plastically deformable saddle.
Article xe2x80x9cQuantitative characterization of 96.5Sn3.5Ag and 80Au20Sn optical fiber solder bond joints on silicon micro-optical bench substratesxe2x80x9d by Rassain M. and Beranek M. W., published in IEEE Transactions on Advanced Packaging, vol. 22 (1999), pages 86-93, describes stress analyses of optical fibre solder bond joints on silicon substrates under thermal cycle loading for a planar and a V-groove bond joint configuration. Results of stress analyses are reported as a function of the distance between the bottom of the fiber and the silicon substrate.
U.S. Pat. Nos. 6,207,950 and 6,227,724 disclose an opto-electronic package using a flexure to align an optical fibre to an edge-emitting opto-electronic element, such as a laser diode. The fibre is attached on the underside portion of the flexure facing the package substrate. The flexure is attached by laser microwelding to a metallic frame, which is attached to an electrically isolating substrate. The laser diode is mounted on a raised platform (submount) made of a high thermal conductivity material, which is attached on the substrate.
The assembly comprising the optical bench mounting the opto-electronic device, the fibre holding and aligning members, including the fibre, and possibly other complementary internal devices or optical components (e.g., lenses) is generally referred in the art to as the optical assembly. The optical assembly can be placed on a thermoelectric cooler (TEC) or on a heat-sink in order to stabilise the laser temperature during operation. The heat produced by the laser spreads through the submount and the optical bench to finally reach the TEC surface. Applicants have found that isotropically uniform thermal diffusion is at least partially inhibited when the laser is placed on a submount, even when the submount is made of a material with high thermal conductivity. They have further observed that thermal resistance of the optical assembly increases with the thickness of the submount.
The optical assembly is generally housed in a hermetically sealed package including a metallic box provided with input/output electrical leads. Presently, 14-pin butterfly packages have become standardised throughout the industry for laser packaging, e.g., for 980 nm pump laser devices or 920 nm multimode lasers for fibre lasers or Raman amplifiers, since they enable customers to source from more than one supplier. These standard packages are usually provided with a nose tube (snout) mounted at the exterior of a package wall, which surrounds a hole in the wall for the insertion of the fibre ending portion into the package. The operation of inserting the fibre through the snout generally takes place after the fibre ending portion has been secured, e.g., soldered, into a support member. Insertion can occur with the help of a fibre loader probe assembly or of a grip assembly, e.g., tweezers. Applicants have observed that handling with a gripping mechanism the support member during insertion of the fibre in the package can lead to the fibre breakage when the fibre is secured in a slotted rod with the fibre facing upwards.
The invention relates, in a first aspect, to an optical assembly for opto-electronic packages comprising an optical fibre secured on the underside of an elongated support member in optical alignment with an opto-electronic device, wherein said support member is affixed to an aligning member, which in turn is affixed to a welding platform. Preferably, the elongated support has a length-to-width ratio not smaller than 1.5, more preferably not smaller than 3. In a preferred embodiment, the elongated support member extends in a direction substantially perpendicular to the upper surface of the substrate by not more than 50 xcexcm beyond the surrounding surface of the fibre.
One of the advantages of the present invention is that the fibre is permanently secured on the underside of a support member that can be aligned in 3D (e.g., x-y-z alignment) before being fixed, e.g., by laser welding, to the aligning member and hence to the welding platform.
In particular the invention relates an optical assembly for opto-electronic packages comprising:
a substrate having an upper surface;
an optical fibre having a longitudinal axis;
an opto-electronic device which is mounted on a first portion of the substrate upper surface and which is optically coupled with said optical fibre;
at least a welding platform placed on a second portion of the substrate upper surface;
at least an aligning member comprising at least a surface for being affixed to the welding platform and an aligning surface;
an elongated support member holding the optical fibre in optical coupling with the opto-electronic device, said elongated support member having at least an aligning surface contacting the aligning surface of the aligning member, the two aligning surfaces being shaped so as to allow relative movement of the aligning member and the elongated support member during an alignment phase, said elongated support member being affixed to the aligning member after the aligning phase,
wherein the optical fibre comprises an ending portion which is secured on the underside of said elongated support member facing the upper surface of the substrate.
In a preferred embodiment, the elongated support member is a planar parallelepiped. In a further preferred embodiment, the elongated support member is a parallelepiped with an axially extended slot, in which the optical fiber is secured with its longitudinal axis substantially parallel to the axially extended slot.
The substrate mounting the opto-electronic device and the welding platform, i.e., the optical bench, is preferably made by a material having thermal conductivity not less than 140 W/mK. Applicants have observed that thermal properties of the package are sensibly improved when the thickness of the submount for the opto-electronic device is reduced to a height smaller than 0.2 mm. Applicants have also found that thermal resistance of the package can be minimised when the laser is mounted directly on the surface of a highly thermally conductive optical bench.
The optical assembly is suitable for being housed in an opto-electronic package, for example a 14-pin butterfly package or a dual in-line package. The optical assembly is particularly suitable for being housed in a thermally controlled opto-electronic package, which includes a cooler, such as a heat-sink or a thermoelectric cooler (TEC).
The invention relates also to method of optically coupling an optical fibre with an opto-electronic device, wherein said optical fibre has a longitudinal axis and comprises an ending portion, and said opto-electronic device is housed in a package housing provided with a hole in one of its walls, said method comprising the steps of:
positioning the optical fibre ending portion on a surface of an elongated support member so that the longitudinal axis of the fibre is placed along the direction of elongation of the support member;
permanently securing the optical fibre on the support member surface;
turning the support member with the permanently secured fibre so that the fibre is positioned on the underside of the support member;
feeding the support member axially through the hole in the wall of the package housing, and
aligning the optical fibre with the opto-electronic device and affixing it to the package in an aligned position.
Applicants have found that by handling the support member with the fibre secured on its underside, the risk of levering off the fibre from the support member is substantially avoided, thereby reducing the risk of fibre breakage.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention.